BART and Amtrak are teaming up on assessing the possibility of building a second Bay Area crossing, one that would give passengers a “one-seat” ride on the Capitol Corridor train from downtown San Francisco to Sacramento.

“That’s the hope,” said David Kutrosky, managing director of the Capitol Corridor.

The idea would be to run two sets of tracks across the bay, either through a tube or on a bridge.

“Either way is fine,” Kutrosky said.

The idea of a second rail crossing to increase BART’s transbay capacity has been around for decades, but adding a second regional rail line appears to be giving the idea new life.

Plus, thanks to the $3.5 billion bond BART measure passed by voters in 2016, there is money available to start making moves.

The first step came in November, when BART sent out to bid a $50 million, 10-year contract for a serious look at the feasibility of a second crossing. The bids are due back this week.

One plan under consideration would be to add a new Capitol Corridor line that would run through the East Bay and then feed directly into San Francisco, possibly into the Transbay Transit Center at Mission and Beale streets.

“That is definitely a possibility,” Kutrosky said.

Another option is to have the train come into another SoMa location, perhaps the Caltrain station at Fourth and King streets, or perhaps farther south.

Either way, the eventual goal would be to hook the Capitol Corridor line to Caltrain’s Peninsula service, allowing for a straight shot to San Jose.

The idea is to “open up a brand-new market by offering a one-seat ride from Sacramento to San Francisco,” Kutrosky said.

Currently, Amtrak’s Capitol Corridor train runs along the East Bay shoreline through Oakland, Hayward and Fremont, down to San Jose. That route would remain.

San Francisco-bound Capitol Corridor passengers boarding in the Sacramento Valley region now have two options: Transfer to a bus at the Emeryville Station or transfer to BART at the Richmond Station.

“Even with these transfers in the East Bay, the Sacramento-to-San Francisco trip is the most popular on the line, averaging about 750,000 trips per year.” Kutrosky said.

The line could also handle high-speed rail.

“That’s kind of the dream scenario,” said Randy Rentschler, director of legislation and public affairs for the Metropolitan Transportation Commission. “But only an engineering study can tell us if that can — or should — be done.”

The specific routes for the various options, along with the touchdown points on each side of the bay and the costs, will all be part of the study.

BART Director Robert Raburn, whose district includes part of Alameda County, where the eastern side of the crossing would likely be located, is already on board.

“It has to be built. It will offer more workplaces, tourist and convention businesses, and growing populations access to high-capacity, 24-hour transbay service, and direct links to California’s other metro areas,” Raburn said.

In the best of cases, construction of a new tunnel is many years and more than $10 billion away. But there is already talk of asking voters to approve a regional transportation megabond that could help foot the bill.

And, as anyone who has followed Bay Area transit will tell you, once something big starts rolling it usually keeps rolling.

Photo: Yalonda M. James / The Chronicle

San Francisco Mayor London Breed delivers her State of the City address at the National LGBTQ Center for the Arts.

On your mark: Mayor London Breed is kicking off her re-election year with no big-name opponent and none on the horizon.

But just to be safe, she has already amassed $240,000 for the November election, according to campaign finance reports recently filed with the San Francisco Ethics Commission.

Records show Breed has raised $271,000 and spent $31,000 so far in her bid for a full term.

A look at the first-round donations showed a healthy cross-section of the city’s economic and ethnic lines, from the $250 donation given by U.S. Postal Service letter carrier Ben Gong to the $500 given by Recreation and Park Commission President Mark Buell.

As usual for incumbents, the donations for Breed’s re-election included dozens of $500 checks from people with real estate, development and other interests that do business with the city.

Breed faced a tough fight in a crowded race to fill out the term of the late Mayor Ed Lee last June, including well-financed and well-known rivals like then-Supervisor Jane Kim and former state Sen. MarkLeno.

This time out it’s a different picture.

“I haven’t heard of anyone,” said the Democratic County Central Committee Chair David Campos, also a former supervisor.

And while 20 candidates have taken out papers for the race, none would be considered a serious threat to Breed.

Still, with the filing deadline four months away, Breed isn’t taking any chances.

San Francisco Chronicle columnist Phillip Matier appears Sundays and Wednesdays. Matier can be seen on the KPIX-TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call 415-777-8815, or email pmatier@sfchronicle.com. Twitter: @philmatier